On a high speed ADC card I was looking at, the connector used was the HPC variety of the Vita 57 FMC. The reason given for using HPC over LPC is that for a 8 lane link an LPC cannot support that many JESD lanes.
However, the JESD standard only requires 2 lines per lane for differential transmission plus the SYNC~ and CLK signals. The LPC seems to have more than enough physical pins, and on the card in question many LPC pins are left disconnected.
So what is the limiting factor that dictates how JESD can be implemented through an FMC connector?